Converting 26.62 NOx µg/L into mM (millimolars)? I have to convert  a whole lot of nutrient data : $\ce{NH_4}$, $\ce{PO_4}$ and $\ce{NO_x}$ data that is in µg/L into mM (millimolars). 

Could someone give me an example for 26.62 $\ce{NO_x}$ µg/L? or 7.73 $\ce{PO_4}$ µg/L? or 14.39 $\ce{NH_4}$ µg/L?

I have a large data set in Excel to convert and send to someone and want to get it right. Any help would be really appreciated.
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