Is Andorra in the EU?
Can I stamp my passport when I enter Andorra to extend my 90 days allowed?
Technically yes, realistically it depends on who is on duty at the border and whether they can be bothered to find the stamp. Andorra has random border checks and tourist "Immigration stamp collectors" looking for that souvenir can get a tourist stamp BUT this is undated. To get an officially dated exit stamp from Spain and then an entry at the Andorra side is not guranteed at all. I do know persons have been successful and I know some who have not been so lucky. If you are entering by bus then the chances are far higher to be checked than by private or hire car, and also by entering on the bus it is far more unlikely that they will wait for you while you chase the officials for that stamp !!
In January 2011 one potential passive resident applicant on a single entry visa was unluckily stopped by a random border check on a bus from Barcelona to Andorra and was refused entry, so beware of the risks of a single entry visa.
I should add at this stage that non EU persons applying for work permits must have in their passport a valid multiple entry Schengen visa, but for passive residency this does not appear to be a requirement for the actual processing of the application once in Andorra, but due to the reasons above any potential applicant should get a multiple entry visa. Once in Andorra it becomes irrelevant that you will exceed the 90 days Schengen or the passport visa as long as you have the residency card once you leave from an EU airport. This as explained below is important as holding an Andorran residency permit exempts you from any Schengen visa or entry requirements ( including transit visas).
So what does the actual law say on Schengen ?
The Schengen Area comprises the territories of 26 European countries that have implemented the Schengen Agreement signed in the town of Schengen, Luxembourg, in 1985. The Schengen Area operates very much like a single state for international travel purposes with border controls for travelers traveling in and out of the area, but with no internal border controls.
Just how do I get an Andorran passport ?
Now when the Schengen Treaty was signed, Andorra amongst others would need to have agreements with Schengen for obvious reasons, such as being landlocked by France and Spain and requiring access to the outside world. As far as Andorra is concerned the agreement was made with France and Spain in 2000 and for those researchers I quote:-
Wishing to regulate the procedures of movement and residence of foreigners traveling to the territory of Andorra;
have agreed as follows:....
...."The system of entry and movement in France and Spain of foreigners holding a residence permit of Andorra is identical to that applied to foreigners holding a French or Spanish residence permit ....."
I have been asked whether arriving in Schengen from outside with an Andorran residency permit at say Frankfurt would be OK.
Unfortunately in practice when travelling to the Schengen states from all over the world, how many airline check in staff, supervisors, Immigration officials have ever heard of Andorra, let alone the Accord with France and Spain !!