No, it's not feasible.
The fundamental problems that prevent this are:
- The Pioneers do not have enough power to operate the transmitter, due to corrosion of the thermocouples
- The Voyagers and Pioneers, even at full power, use very low power transmissions
- The Pioneers would need to be able to receive and obey instructions to aim for a Voyager
- The craft are not significantly closer to each other than to Earth.
- Reprogramming of Voyager would likely remove its ability to use its own instruments
1 - Power
Pioneers 10 and 11 both have shut down due to insufficient power to operate both the computer and the radio at the same time.
2 - Low Power Transmissions
NASA uses huge antennae to capture the weak signals from the probes. The smallest dishes used are around 10m across, and in recent years, they've been using the larger (around 30m) dishes.
3 - Finding Voyager?
The Pioneer probes, in order to make this work, would have to be "effectively closer" to the Voyagers than to Earth, be able to power up, and be able to aim at the Voyager probe being used.
This means the Pioneer would need to receive, process, and obey instructions to point at the Voyager, and the only way such a relay would be known would be after it worked. But this would require that we be able to send those instructions in the first place, and if it's operational at all, it's aimed at Earth.
But further, it's not just the Pioneer that would need to point... the Voyager would have to aim at the Pioneer to make it work. And that means it would not be aimed at Earth. See #5...
4 - Range
Radio follows an inverse square law. In order to pick up the 8w signal from 36 AU, NASA needed the 36m antenna on the ground. Voyager's antenna is under 3m.
Pioneer 11 for Thu 13 February 2014
Right Ascension: 18h 49m 45.2s Declination: -8° 48' 07.3" (J2000)
Sun dist: 13,202,867,763 km [11.4 km/s] Earth dist: 13,319,310,346 km [27.4 km/s]
Constellation: Sct Magnitude: N.A.
Voyager 1 Spacecraft, ephemeris for Thu 13 February 2014, 00:00 UTC
Right Ascension: 17h 11m 59s Declination: +12° 02’ 31” (J2000) [HMS|00:00:00|Dec]
Distance from Sun: 18,977.42 Million Km Distance from Earth: 19,021.59 Million Km
Magnitude: N.A. Constellation: Oph
Voyager 2 Spacecraft, ephemeris for Thu 13 February 2014, 00:00 UTC
Right Ascension: 20h 00m 25s Declination: -55° 57’ 34” (J2000) [HMS|00:00:00|Dec]
Distance from Sun: 15,563.42 Million Km Distance from Earth: 15,663.78 Million Km
Magnitude: N.A. Constellation: Tel
This gives us a minimum of 20° difference in angle, and the distances can be roughly calculated based upon a 100AU triangle...
Given 100 x Sin(20°) that's 34 AU and change. Voyager's dish isn't big enough, at 3.6m; it pulls in roughly 1/100 the signal that the 36m dish on earth would. (other factors make this extremely simplistic an analysis, but establish the point well enough.)
Simply put, the receiver is not sensitive enough, and the antenna gain not sufficient, to enable voyager to notice the energy from a pioneer transmitter.
5 - Reprogramming Voyager
Voyager probes don't have a whole lot of memory. And much of that memory is of dubious reliability now. The reprogramming would need to revise the whole mission programming, even if Voyager could "hear" the Pioneer 11 transmission.
This would scrap ongoing data collection, possibly cripple the Voyager, and still not assure that the data would relay.
Conclusion & Discussion
Given that any one of the five reasons is sufficient for a no-go on such an attempt, it's a fairly certain thing that no space probe is going to be used to contact the Pioneers.
Whether a later probe might be used to relay to/from the Voyagers is another matter.