The line is outside or not part of the drawing view and hence you can not add dimension between them.
Select the line and do ctrl + x (cut) or ctrl + c (copy). Now select/highlight the view and do ctrl + v (paste). And then you can select both of them.
you can't add dimensions between two features that are not in the current sketch. you can dimension from a feature in your sketch to a feature elsewhere on your part.
in addition to Deepak's suggestion you can also use construction lines to bring something into your sketch. just make it colinear or coradial to the external feature.