Learn Finnish: Nämä ovat osa-aikaiset työntekijät. - These are the part-time employees.

In Finnish, using the nominative plural here makes the group definite, translating to "the part-time employees" (a specific, complete group). If we used the partitive plural (Nämä ovat osa-aikaisia työntekijöitä), it would mean "These are part-time employees" (an indefinite group).

Since this lesson is about the tulla (to become) construction, how would I say "These will become the part-time employees"?

You use the elative case for the starting state (näistä) and the nominative for the result. The sentence becomes Näistä tulee osa-aikaiset työntekijät. (Literally: Out of these will come the part-time employees).

How is the adjective osa-aikaiset formed?

It is a compound word. Osa means "part" and aika means "time". Adding the suffix -inen turns it into the adjective osa-aikainen (part-time). Because it describes a plural noun in the nominative case, the -inen ending changes to -iset to match.

Why does the word työntekijät have an n in the middle?