No, the kernel.sin not contains the baseband.
If you want to flash the baseband with custom kernel, just insert the amss.sin and the amss_fs.sin into the kernel flashtool package (xyz.ftf).
The amss contains a microkernel and a small RTOS for the embedded modem processor (a separated arm9 cpu in the msm7x27 soc), this is controll everything radio related thing. It also controll the hexagon dsp and few other things.
So, the standard kernel package contains only two file:
If you want to flash the baseband with the kernel (in same time), you need kernel package with these files:
The flashtool packages is simple zip files with changed extension (.zip -> .ftf), easy to add the baseband files.
The sin files contains every needed informations (partition starting address, type, etc), so nothing need to edit, just add the baseband files from an official rom.