Thank you for your interest in the reface CP. The reface's specially-designed mini keys are extremely playable (unlike most minikeys) but if you need larger or more keys, yes, you can. If your keyboard has traditional MIDI IN and MIDI OUT connections, it can connect to the MIDI breakout cable included with the reface CP using standard MIDI cables.
If your keyboard has a Class-Compliant USB-to-Host connection rather than MIDI IN and MIDI OUT, you can use a USB-to-Host > MIDI converter, similar to this model from Kenton:
Or the mio4 from iConnectivity:
(Some pre-configuration of the converter using a computer may be required)