As the ISS has shown the ability for human habitation for extended periods, might we be able to add a propulsion unit to the ISS that would be capable of providing delta-v to send ISS to a martian orbit? Probably a low acceleration system that would allow for the maintenance of structural integrity would be needed.
A supply module with supplies sufficient for the Mars transit period would have to be added with the necessary supplies for the mission. If this presents structural or other problem, perhaps there could be resupply missions that could occur in Mars transit, or perhaps supply repositories placed in path of the ISS.
Likely, its power generating system like solar panels might have to be upgraded to meet internal energy needs at increased distance from the Sun.
Now a bit of an editorial for which I have been chastised in the past. This approach would take advantage of our proven capability of assembling a major structure in orbit using many lifts of smaller modules, rather than the one shot approach that NASA is employing in developing larger lift capability and the one shot to space approach of the Orion unit. Orion must fit within an envelop needed for launch.