ASM

"How can I add an asembler code to C project?"

Exactly the same way that you add a 'C' file to a project - but choose "ASM" as the type instead of 'C'.
uVision automatically recognises the file type & uses the appropriate translator, syntax colouring, etc.

You have read the uVision "Getting Started" Guide, and worked through the examples in it, haven't you...?

"How can I add an asembler code to C project?"

Exactly the same way that you add a 'C' file to a project - but choose "ASM" as the type instead of 'C'.
uVision automatically recognises the file type & uses the appropriate translator, syntax colouring, etc.

You have read the uVision "Getting Started" Guide, and worked through the examples in it, haven't you...?

Note that there's a minor bug in uVision in at least some of the "add file" dialogs. The drop-down list doesn't always have the right file extension. It looks for "*.a" or "*.asm" instead of "*.a51". I just change the search to "*.a*" once I've selected the file type from the drop-down list.

uVision uses the file type to know which translator (C compiler, assembler, librarian, etc) to run. You can change this type assignment by right-mousing on the file and choosing the "Options" menu.