"Is it possible to use declarations from C module in assembler module?"

'C' and Assembler are completely different languages with, obviously, completely different syntax & semantics. It would be a Big Surprise indeed to find 'C' syntax working in Assembler, or vice-versa!

It may, however, be possible that your particular assembler has certain specific features designed to be compatible with your particular 'C' Compiler - you will have to read the manual to find the details!

I believe that Keil do claim that their A51 supports (some of the) C51 pre-processor facilities, but I also seem to remember a number of recent posts on their forum from people having problems with this!

Note that the 'C' preprocessor is purely a text-processing utility; it has no actual "knowledge" of the 'C' language.
Therefore, if you have a standalone 'C' preprocessor, you could create suitable "universal" macros which can be preprocessed into either Assembler or 'C' definitions - and you could pass your Assembler files through the preprocessor prior to Assembling this.
(a previous Client used a similar trick to manipulate Linker control files!)

I think you can obtain the Borland 'C' preprocessor for free?