"Previo[s]" followed by a noun refers to some condition that needs to be fulfilled for whatever is mentioned in the main clause of the sentence that follows. It may refer to something that *needs to happen* for something to take place in the future, or - as here - to *a condition that has already been fulfilled* in order for an event that has occurred (here, the invitation to visit China).

In short, the "convenios" here have already been signed. This, at any rate, is my understanding.

I suppose that what is being referred here are visas and logistical and financial arrangements for the lectures that the author of the text is to give.

Veronica has the right idea here, but "subscribe" is overly formal here (given that this is not the text of the actual contract, but simply a reference thereto).

I think that the impersonal rendering I suggest works best here. It is understood that the agreement in question was between the lecturer and the party or parties requesting the author's presence.