There are several ways of doing this. Firstly if the earlier kernel had modem support which isn't in the new kernels you can recompile the kernel source to add the support. Yes you can have up to 8 versions of linux all available through a boot menu(normally the boot menu only holds regular load/ safe mode load/ and (opt) new-build load). I am a little puzzled about it not having support for your modem though. Linux as a rule supports all internal and external modems with the exception of winmodems. Typically, legacy types are jumper configured and setup with modemtool, pnp types typically need set-up with isapnp then modemtool. And as far as operation goes, almost all modems now a days use hayes compatiblility in modem mode so only voiceand fax modes really need special drivers.
Now that i have shot my mouth off, there was a flurtation with modems that required dos bootloaded programs in order to initiatize but these were to my knowlege all legacy types.
Hope some of this helps
BTW: you might try reading the modem howto's they have extensive details on modem work arounds.
rick