Installing a Yamaha temp indication

Maybe I should not get involved but I was wondering if it's possible to add a engine temp gage . Is there away to tap into the t temp. sensor on the engine somewhere else ? My engine's are Yamaha 2004 150 Z150TXRC HPDI

No, you can't tap into the temp sensor. Those sensors work on resistance and only turn the alarm on when the engine gets to a certain temp.
A temp gauge is installed where it's sensor is in contact with the water as it leaves the engine. It has a bimetallic sensor on the end.
Some engines have a blank plastic plug on top of the exhaust box next to where the pee tube comes out.
If you find it, remove it and run the engine with water on or in the water to verify it is in fact the right plug.
If it is you can install the temp gauge senser there and run it to your gauge on the dash.

If you could find a temp gauge that reads off resistance instead of actual water temp (and of course it would need to be the same resistance specs as the installed temp alarm) it should read the resistance of the temp alarms and work ok.
I would probably install a water pressure gauge instead of a temp sensor. The pressure gauge is going to read regardless of water temp of course, where as a temp sensor is going to read at different temps while the engine runs depending on how warm/cold the surrounding water is. The pressure gauge would give you a better indication of the status of your water pump after you run the engine enough with a new water pump so you can see the "normal" max water pressure.

The draw back to a water pressure gauge is this; some impellers are small diameter (which mean they work by building up pressure) whereas other impellers are large diameter (which mean the work on volume of water). My 99 Yamaha SX150TXRX OX-66 uses the larger diameter impeller so of course it is designed to produce more volume of water vice higher pressure of water. Eitherway, you would be able to gain a sense of how the water pump is working.