That violin mic's 1/4 inch TRS plug almost certainly carries the balanced (signal "hot", signal "cold", and shield) output from some type of balanced-output microphone ...and that mic is also almost certainly low impedance, (whereas "guitar pickup" tip-sleeve-equipped gear is nearly always high impedance) hence successful operation with a G30's beltpack is most unlikely.

It's hard for us to give you any more help than that without a bit more detailed information, but I'll try anyway, based on an assumption that you may be planning to simply swap from your double bass to a violin, with both instruments intended to be alternately re-enforced via the same (existing) receiver.

Unfortunately, the best way to do this involves the additional outlay of around $200 USD for an additional Line 6 TBP12 or V75 beltpack which has a multi-pin input connector, (known as a "TA4") that facilitates the options of either impedance and/or the DC powering of mics via different connections at the beltpack-end of cables.

Here's the relevant page from one supplier's catalog with details of such a beltpack: http://www.sweetwater.com/store/detail/TBP12G/

Once configured to operate in its "RF1" mode, channels 1 to 6 of the twelve available in that beltpack correspond (and will succesfully work) with the six of your existing receiver.

The quickest (less than 30 seconds) and simplest way of then swapping  between instruments would then be to have both instruments' transmitters tuned to the same frequencency as that of your Relay receiver and for you to switch off the "outgoing" instrument's beltpack prior to the swap ...and only then switch on the "incoming" instrument's transmitter.

You'd also need something like this http://www.sweetwater.com/store/detail/PWP047BB/ for you to connect your intended violin mic to the additional beltpack's input via a suitable TA4F adapter cable, such as this one http://www.sweetwater.com/store/detail/WA310/.

If your intended violin pickup device requires also requires DC powering, its manufacturer may very well be able to supply it already terminated with a TA4F plug instead of the TRS jack plug, which would also be a whole lot neater.

Otherwise, even more expense and bulk would be involved as you'd also need an external battery supply, such this one http://www.bhphotovideo.com/c/product/403002-REG/Denecke_PS_1A_PS_1A_Portable_Single.html.