skbuff size

Hi,
I'm writing some code that works with skbuff structs (the structs that
represent packets) in kernel mode. I get skbuff structs from the IP layer
of the host and, before sending the packet on the net, I add an additional
header. The default available space at the beginning of the skbuff struct
is not enough large to contain the extra header, so I need to copy the
skbuff in a larger skbuff (therefore losing time).
Where can I set the default available space at the beginning of a skbuff? ù
I think I may have to modify the kernel sources (and that's fine), but I
don't know how.

Thanks for your help.