Encoder/Drive for Servocity 0.5rpm

Hello, I bought from Robotshop, Servocity 0.5RPM gearmotors and I was wondering if I can add an encoder to them. Any suggestions? Also, which drive should I get for them? Many thanks, Philomene

Could you give me link to the product?
Is there a rear shaft on it?

https://www.servocity.com/0-5-rpm-gear-motor