This is a compound formed from āgāmin and karman, it is a subtype of tatpuruṣa compound, called karmadhāraya. Āgāmī ca karma ca is the resolution, karma that is to come /accrue. Āgāmin, the substantive is derived from the verb gam (1, P) with the prefix ā meaning “come” and the suffix in.
Technical: ā + gam + ṇini = ā + gām + in = āgāmin. The ṇ code changes the a of gam to ā, called a vṛddhi change, the final vowel code i of the coded suffix ṇini does not have any impact. This substantive is in neuter gender.