It is not mandatory to gain maintenance experience in a Part-145 organisation. According to the AMC 66.A.30(a)(4), aircraft maintenance experience gained within different types of maintenance organisations (under Part-145, M.A. Subpart F, FAR-145, etc.) or under the supervision of independent certifying staff may be accepted by the competent authorities. This means that the aircraft maintenance experience may be accepted by the competent authority when such maintenance is performed in a maintenance organisation which does not necessarily hold an EASA Part-145 approval.
Furthermore aircraft maintenance experience gained outside a civil aircraft maintenance environment may include aircraft maintenance experience gained in armed forces, coast guards, police, etc., or in aircraft manufacturing. However, it is on the competent authority to evaluate whether this experience is acceptable.
Consequently, please contact the competent authority where you intend to apply for a licence, in order to check whether the basic experience would be acceptable.
See Part-66.A.30 and related AMC/GM.