"with C language but I'd like to know if it is possible to include assembly code"

most 'C' compilers provide some means to mix assembler into 'C' source - this is all non-standard, so you'll have to read the manual for your particular compiler.

As for calling between 'C' and assembler - of course you can!
You can do anything in assembler, so all you need to find out is the Calling Convention of your 'C' compiler - read the manual - and implement that.
The Keil C51 manual has a section specifically dedicated to this.