In a word, No. You can't just put R134a into an R12 system, it's not compatable with the oil in the compressor, plus R134A isn't compatable with R12, which means you'd have to recover/remove all of the old R12 before you could even think of converting it to R134a,.

There are several substitutes for R12 on the market, ( Hot-shot/R414A/B. R409, etc.) 95% of which you need a licence to purchase legally, but there are some available such as Freeze12, Auto-12. etc that you can buy online without a licence, which are more or less "drop-in" replacements for R12.