I'm afraid it's not possible without a lot of work, and maybe not at all. Cab files contain both program files and installation instructions, such as creation of reg keys or shortcuts.
The biggest problem is setup.dll. Some CABs use it to do some extra work during installation. You can't combine 2 CABs that use separate DLLs unless you write your own DLL that does the work of both.
There is a program called CAB manager (http://www.ocpsoftware.com/products.php?nm=cecabmgr
), which allows editing of existing CAB files.
But I think your best bet is googling for a PC based installer, which can handle multiple CABs. I saw a free one once, but unfortunately can't remember it's name.
Hope that helps, good luck!