The answer is yes, it can used to re-boost the ISS.
According to the information in spaceflightinsider.com and in rocket.com Aerojet Rockedyne site in the RCS engines paragraph, in a declaration of Terry Lorier, Aerojet Rocketdyne’s CST-100 service module propulsion system program manager, are confirmed the capabilities of Boeing CST-100 Starliner spacecraft.
Propulsion system of CST-100 includes different types of engine thrusters with different roles. In spaceflightnow.com are explained what includes the propulsion system.
CST-100 has four 39700-pound thrust (176.6 kN) launch abort engine (LAE) for the CST-100’s ; 24 orbital maneuvering and attitude control thrusters (OMAC) 1500 pounds of thrust (6.67kN) each, for launch abort attitude control, stage separation functions, large orbital maneuvers.
Also are included 28 reaction control system engines (RCS), each of them with 100 pound thrust (0.44 kN) using similar technology to other engines in CST-100. RCS role is to re-boost international space station (ISS). Let say that even the above engines the bigger ones, could be used sometimes to help for re-boosting faster the ISS if there is enough fuel in their respective tanks. But with or without them, the RCS can accomplish the re-boosting procedure for ISS.
This hasn’t to do with your question but I see that you have written Dream Chaser would not be able to re-boost. Actually in sncspace.com Sierra Nevada Corporation (SNC) site and in this article of spaceflightinsider.com, is confirmed this capability. In the official site of SNC is included also the info for cargo and crew Dream Chaser similarities and differences that you have asked in another question. Anyway for the part “engines are blocked”, it could be if we are speaking for the crew version. But at least for cargo version, now I am just supposing that since info said that can re-boost, probably this could happen because in spacecraft is attached the cargo module, and when will be attached to ISS distance between Dream Chaser and ISS module will be considerably bigger and its engines could be lower as level. Probably Dream Chaser cargo and crew roles could be the same as Progress and Soyuz spacecraft situation for re-boosting procedure part.
Anyway current NASA’s request for the role of this spacecrafts CST-100 and Dream Chaser, is to accomplish crew and cargo missions respectively, but if sometimes it will be necessary to re-boost ISS each company of this two spacecrafts has confirmed this capability.