GZIP + access and modify extra field usage

[yperl]

Hi list,

I'd like to use the "EXTRA FIELD" feature of GZIP specification to store 
in the GZIP header itself,
the size of both the "compressed" and the "uncompressed" data.

The only theoritical unfinished solution I come up with is to:
1) compute the actual uncompressed file size
2) compress the file and compute its new compressed size
3) try to use these two values (i.e. compressed uncompressed sizes) in 
someway
and manullay rewrote the GZIP header entirely with the EXTRA FILED set 
... (don't tested yet).

Is there any way offered by ZLIB to access the GZIP header and modify it?
This will help me a lot to achieve my aim.

How is the best way to achieve that please ?

N.B: any source code will be very welcome

Thanks in advance.

cheers
Younès