The PRO-164, Radio Shack's model for the PSR-300, uses an older scheme called "bank technology" to organize channels, frequencies and talk groups. Generally speaking, you dedicate one bank to one trunked system. You certainly can mix the control channel frequencies with conventional frequencies, but it gets complicated. Talk groups, if you chose to use them, are programmed separately and there are places to program them for each bank.
However, there are a lot of cautions with using this radio, as some have mentioned. If the system you want to monitor is analog voice, supported by the radio (MOT, LTR, EDACS) and not subject to re-banding, great. The one last system that fit this mold I was monitoring just switched to P25, which is digital, and can't be monitored by this radio.
So before figuring out whether you can mix conventional and trunked, it's important to know what you specifically want to monitor and if this radio can even do the job. Welcome to Radio Reference!