Converting 26.62 NOx µg/L into mM (millimolars)?
I would make it easy for yourself and convert your µg/L to g/L. You can then divide it by the formula weight to obtain the molarity (in M). After this it is just a simple conversion to mM.

I suppose in Excel you can create a new column and use a formula something like this:

=((CELL/106)/Formula weight)*1000

So for your example of phosphate:

=((0.00000773)/94.97)*1000

=0.00008139 mM // 8.139x10-5 mM