SDCC (and I assume also other compilers) consists of layers of software. You can think of it as gradual translation from language to language (plos the C preprocessing, which grossly is macro expansion and conditional branches clearup), performing various optimisations at each step. Maarten and Frieder would perhaps have more fancy names for each step, such as frontend and backend.
It is in theory possible to replace the first "translation", and it is made easier by the fact that even for the C "frontend" unified tools are used (flex and yacc/bison). Nevertheless, there are differences between the two languages so that the some of the subsequent layers would be affected, too.

Indeed, this approach works and is used in a Pascal port of gcc, google for gpc...

JW