You can't.

Modems in linux usually use kernel modules as their drivers - and you'll be very lucky to get one of those in an RPM!!

Secondly, the HSP modems are (as far as I know) mostly "winmodems" which do not work in Linux, or work only marginally.
You'd be best searching www.google.com for your specific modem make, model and manufacturer, to see if there are any drivers that offer support for your particular modem.
Real modems integrate with linux more or less seamlessly (kernel modules for ppp may need to be installed if you didnt' compile them into the kernel, but a simple:
insmod ppp
will achieve that.
You're probably best buying an external 56k serial modem (Not a USB, a lot of those are Winmodems as well). As long as it supports MS-DOS it'll work in linux (MS DoS does not have winmodem drivers, and so a modem which works in MS-DOS works in linux). You're best looking for an external 3Com or US Robotics Serial 56k modem, as they are very likely to be hardware modems.