Electroplate a circuit onto fr4?
No, that's not going to work.  Even with pre-clad board, having the copper stay adhered to the substrate during soldering can be a problem.  Your plated-over-carbon won't stay stuck at all.
Some commercial board production methods actually do involve electroplating to increase thickness, but they do so starting with a copper cladding, not something else.
If you really want to try something DIY, try heat transferring laser printer toner to serve as an etch resist from flimsy &quot;junk mail&quot; paper onto copper clad and then soaking the paper to remove.  It's tricky to get right, and messy, but with care one can achieve 8-10 mil (0.2-0.25 mm) traces for TQFP's.  Really it's only justified in a &quot;need a simple prototype today&quot; case - otherwise quick turn prototype fabrication makes more sense.