I had a friend ask me this, and i had to say...I have no idea.

I have a 2008 8 HP Mercury 4 stroke outboard. It is a manual start, stick drive, nice little motor.

It is not currently rigged up for this, but is there a mod or kit that you can add to such a motor (or more correctly pay someone to hook up) to hook it into the battery to charge them while you are motoring around?

I would guess no, but I have to admit I know jack about it.

This doesn't directly answer your question (I don't know the answer) but it'd probably cheaper and easier to buy and stow a second battery.

Here is the specs on that motor:

http://www.mercurymarine.com/engines/outboards/fourstrokes/enginesetfour_specs.php?ID=68

It is and can be done. Your local dealer should give you the details.

As mentioned, you will need to check with your dealer.

Being manual start, your motor may not even have a charging system.  If it doesn't, it would probably be cheaper to trade the motor than to try and add one because adding one ain't gonna be cheap.

Would it even throw out enough juice to really charge the battery? :-/

I have no idea. It is not a big deal for me. I realy like the motor and I do have a 2 battery setup already. Course if I could get the motor to charge the battery, that would only serve to increase my range and time on the water.

So not a biggie but if I can extend my fishing time easily, I am always game.

The "available" 6 amp alternator will recover 6A at 12V of battery usage by running the outboard for the same length of time you discharged the battery at that rate. 6A may be the lowest speed setting of your trolling motor.