Can one replace/add a ship part in orbit? [duplicate]

Yes. To some extent, that is.

Without mods you can use docking ports and some form of your own standard for modules. You can either equip each module with RCS and probe core or use small space-tug for assembly. If your reusable ship lacks docking port, you can always brutally attach new modules with Advanced Grabbing Unit.

If it is not enough, there is gallery of mods you can use:

If you want to avoid "cheating" KIS and KAS are cleanest solutions.

Kerbal Inventory System (KIS) allows you to even build rover on the moon. There are problems with large and/or heavy parts, so ground testing is advised. Also, this mod comes with explosive charges for (not) precise part removal.

Kerbal Attachment System (KAS) is complementary mod to KIS, adding winches, fuel lines, ports and other fun stuff. And space harpoons.

There is also Infernal Robotics with space shuttle style robotic arms for assembly. (This mod is not yet updated for most recent KPS 1.0.5, also it tends to be really infernal in usage).

If you want to go further toward Sci-Fi you can try Extraplanetary Launch Pads it brings capability of orbital (and extraplanetary, of course) ship assembly, but it might be unfit for Career mode.