I checked that previous has not plural form so I can use it in both cases, singular and plural forms, cannot I?
For example: I gained BPR skills in all my previous experiences.
I think that even experience which has not plural form, in this case I hae to use it putting s at the end of the word.
Is it correct?
Previous is an adjective, not a noun, so it doesn't have a singular or plural form. As for the noun experience, it can be count or non-count:
Ex: I had many horrifying experiences as a pilot.
Ex: I need work experience in that field to get that job.