It is possible on paper but heavy in practice. Registering an imported car with Portugal's IMT means:
- A Portuguese tax number (NIF) and an address in Portugal
- A customs vehicle declaration (DAV) filed within 20 working days of the car entering Portugal
- Paying the vehicle tax ISV, calculated on engine size and CO2 (used EU cars get a reduction by age)
- A Portuguese inspection, then requesting registration from the IMT within 60 days of the customs clearance
- After that, the annual circulation tax IUC
For someone who spends a few months a year in Portugal, that is a lot of cost and paperwork for a car that also needs to be legal elsewhere.