is there anyway to set it up as not only a music streaming device, but able to switch to a normal radio using an antenna with say a button press?
Yes this is possible, but you will have to install in an FM Tuner module which will act as a second audio source.
while you're at it better add a third sound source called "AUX"
Thus your sound sources will be
a. Raspberry Pi (Zero W) with pimoroni pHAT DAC, Volumio music app
b. FM tuner module
c. AUX input (from RCA jack at the back)
you may add one more if you like.
the audio outputs of the three will be connected to a selector switch (there are digital selector switch available) or re-wire the push button switches in front panel to act as source selector switch.
the output of the selector switch will be connected to the audio input section of the amplifier.
the amplifier output will be connected to the speaker.
Note: you may also need to install a "Ground Loop Isolator/Noise Filter" between the selector sw and the amplifier. To get rid of the awful "HuMmmm", "Pops" and "Crackles" associated with connecting audio sources from 1 power source. Its not that expensive. Cost me less than USD1 for one unit.
FYI: With the volumio system on the RPi, you may be able to tune in to internet radio stations (there are quite a lot to choose from)
..outta curiosity why keep the crystals? What else could I use them for?
That is if it were me
.... I'd keep the crystals, maybe I might
have use for those in future projects...
but if you think you wont have any need for those crystals, then you may get rid (or maybe sell off) those as well.
Looking forward to your progress in your build....have fun!