Of course you can. Paul McCartney actually gigged with a guitar strung with 4 heavy strings (for a very short time) while he was learning bass...so why not? Yeah, a real bass is different, but when you're starting out and just figuring out fingerings and approaches, a guitar will do fine. Of course McCartney graduated from that to a short scale Hofner (first borrowing Sutcliffe's and then buying his own iconic violin model), which would be a natural progression. And The Presidents Of The United States' "Bassist" uses an instrument that was originally a guitar converted to use bass strings, and he sounds just fine. It's all good.

