Can I use cartridge ink for dip fountain pens? I understand that ink in fountain pen cartridges is a bit thinner. Can I still use it for dip pens if I squeeze it out of the cartridges? I have an excessive amount of this ink, and might not go back to using cartridge fountain pens. 

If it doesn't work right out of the cartridge, can I mix something in it to make it usable? Or evaporate it somehow?
Due to the many different kind of quill pens nib designs, I don't think any definitive yes-no answer is possible here.

Generally, yes, ink will flow.

What kind of writing you want to do will have much influence on the decision about whether you use it.  Probably the best choice is to experiment by having a few controlled dips, and creating a few controlled strokes.


Dip to a fixed-depth, and the draw the pen nib down a page until it is exhausted.  Make multiple strokes as needed.  Repeat this for varied dip depths.  
Then try letter-writing.
And / or, try other common nib-strokes related to the kind of purpose you will employ the pen.


If you find a writing style comfortable enough — problem solved.