Contact microphone preamp

I need to set up a contact microphone system to record through a wall continuously for a little while. Most of the wall microphones sold online are battery powered. I recently purchased the following two models:

Nikkei Acoustics
http://nikkei-acoustics.com/


Unbranded F-999B
**broken link removed**


It is unbranded, but seems to be a rough copy of this one from Sun Mechatronics: **broken link removed**

The Nikkei Acoustics model originally ran on a standard replaceable 9V battery. I don’t have a photo, but the battery setup was similar to this:
**broken link removed**

At my request, its eBay reseller was able to solder a DC connector onto it for me, enabling it to run off an AC-DC mains adapter instead. Its internals are shown below:

The case disassembles easily with a hex key.
Ignore the battery taped on top.



The circuit board then slides out manually from the case (it is held by friction).


The DC connector is shown here. Its cable is fed through the circuit board to the other side, and held in place with a dab of hot glue.



Here is the other side of the circuit board, showing the actual soldered connections of the DC connector’s cable.



I want to know if it is possible to fit a similar DC connector to the F-999B model. Unlike the Nikkei model, which ran on a standard replaceable 9V battery before the modification, the F-999B model runs on a fixed rechargeable internal lithium ion battery. Its internals are shown below:





The F999-B has better sound quality, because the preamp is better (although the actual microphone of the Nikkei is better). If the DC connector modification can be made to it, I want to use the F999-B instead. It did actually come with an AC-DC adapter plug, which the seller told me could be used for continuous mains operation. After reading the manual, it became clear that this was inaccurate. The supplied adapter is only for charging the internal lithium battery, and the manual says the unit must be turned off during recharging. Hence, I need to remove the lithium battery completely, and replace it with a soldered DC connector like on the modification shown above. Unfortuantely, I don’t know what DC voltage to apply once it is complete. The lithium battery is specified as 3.7V, but (if I understand correctly) this can correspond to higher voltages in normal DC batteries.

I don’t know how to solder myself, so I need to know:

1. If the modification is possible
2. Who could do it (Electronics shop? Hobby shop? etc). I’m based in London but could post the unit out for the job and then pay for return if the shop is far away.
3. What voltage I should send to the unit after the modification is complete.

If it is performed successfully, I will need another 3 or 4 done.

Thanks for your time...
...and apologies for my cluelessness.

1,2 it's fairly simple, do you have a 'place' for the jack? with enough clearance? or use an inline jack?

3 - 3.7V but I doubt you find a PSU giving that. You could use a 5V PSU IF you wired in 2 diodes [1N4001] in series between the jack and pre-amp. Although the pre-amp, I would think[!] can handle 5V.