If I create a Sitecore package for some Dictionary items with Sitecore 7.5, can I import the package to a site running on Sitecore 8.1 Update 3?
If you are talking about following templates (not any custom that I am not aware of):
Then the answer is YES.
In one open source module we haven't change many dictionary items for a long time (5 years already) and everything is fine..
Sitecore Package structure wasn't changed since Sitecore 7.5 for sure so I don't see any contraindications.