If you can't get a spec from Alinco, the other option is to look at what's typical for an electret mic capsule...http://www.jlielectronics.com/spec.htm
Not much variation in sensitivity numbers and with a rating of +/- 4 dB I'd say they're all about the same. Which should give you a ballpark level and impedance to design for. Then you can add a potentiometer with a DC blocking cap and set the source level to wherever it needs to be.
Also....... If this is a digital interface, this is what Alinco suggests for packet: http://www.alinco.com/pdf.files/Tech/Packet/DR-130T_PKT.pdf
Hm, the sensitivity is in V/Pa. This website
suggests, for human voice levels, 50-60 dB = 2-20 Pa. Sensitivity of -42 dB would then give 0.1-1 mVrms. Looks like the 2 estimates so far line up at about 1mVrms. Seem reasonable? Maybe could add the trimmer as you suggest to give something between 0.5 and 2 mV.
BTW, this is for a bluetooth radio. Should be able to control the radio from the computer or smartphone without any wires