Is it possible to put an existing mod (that didn't use mcreator to make it) into mcreator to continue the mod?

Sorry for asking again, i know this is classed as scam but right now i am really desperate for answers. I love flash and the only mod that really adds flash and stuff for 1.12.2 (speedster heroes) is dead. I'm not very experienced with coding and i already tried to copy and paste code and reroute the stuff in it but the build keeps failing to build because of some so cant export it and test it. If i could get answers soon i would really appreciate it. Thanks for reading and i hope i get some answers soon. (Also They did give us the source code if we wanted to continue it with something like IntelliJ but whenever i try to open it there, it always says it failed to build)