Design for repeated measures and behavioral variable

I have resting state data for a large group of subjects investigated 1, 2, or 3 times.
I would like to model how a variable # impacts rsFC.

If I enter the following for 50 subjects

Session1Session 2Session3

SubjectAScan_1Scan_2
SubjectBScan_1
SubjectCScan_1Scan_2Scan_3
...
SubjectYScan_1Scan_2Scan_3
SubjectZScan_1



Can I then enter the measure  # at each scan somehow as a 3X50 matrix such as
#1#2#3
SubjectA2535 15
SubjectB33nana
SubjectC441129
...
SubjectY34114
SubjectZ22nana

where #1, #2 and #3 is my measure at the three time points


My end goal is to see how # may impacts rsFC

What should my contrast variable look like?
 Scan_1, Scan_2, Scan_3, #1, #2, #3
000111

will this give me the variance between scan sessions that is explained by variance in #, whole accounting for both the within subject and between subject design?
and how do I handle the cases with only one scan?

thanks

Clas