Just out of curiosity, why not use the 4th pump for glut. (excel). Or are you using it for something else. I think with the jabao dp4 you can program each channel individually so you should be able to have the 4th channel run every day.
The reason I wouldn't mix excess excel into your mixtures is well, reactions. Chemistry is messy and excel (glutaraldehyde) is an organic molecule, which you now will be mixing with your salts. My biggest concern would lie with it reacting with you KH2PO4 in the process of pinacol coupling. This obviously depends on the pH of your water, hard water (high pH) won't run into this issue, low pH will. But that is just one of an inumeral amount of reactions that could occur, will they occur.... I don't know I havent tried it myself, like you said some people add small amounts.
2nd concern would be that you should make sure that you either change your dosing amount or you reduce your water for your mixture, because when you add the excel you will be diluting your dosing mixture. eg: if you have 500 ml of solution and would dose 50ml a day, but then you add 500ml of excel to that you should really dose 100ml a day to get your ideal amount of nutrients (which may be too much excel depending on the size of your tank).
Honestly it will be easier if you can set it up on your 4th channel. If not it will be harder, possibly possible, but harder for sure.