The short answer is no, you can’t.
The longer answer is, maybe, but probably not. First, you’d need to supply your own audio, since most customs in the C3DB have encrypted audio. Second, you’d have to bring your own multitracks. Only a tiny percentage of popular songs have multitracks publicly available, and most of those are on sites we can’t and shouldn’t promote here. There are tools for separating stereo audio into some isolated tracks, and sometimes the separation is pretty darn good, but it’s never as good as having true multitracks in the first place, and a lot of instruments remain glued together. Spleeter and demucs, two of the most popular options, will separate vocals, bass and drums, and Spleeter will (try to) separate piano also, but everything else is left as is.
Of course you’re welcome to try, and this is one way you could also get familiar with the tools for creating RB3 customs. But if you’re hoping for some single-button thing that turns a stereo custom into a multitracked one, there’s no such thing.