harry potter - In Quidditch could the Seeker play other positions?

I don't recall it being mentioned in any of the books, but could the Seeker in Quidditch switch to being a Chaser if needed?

I always thought it was odd that catching the Snitch would net your team 150 points but end the game no matter what. It seemed to me that the seeker would be a more valuable player if they could net their team 150 points but not end the game if they were behind. However, the rules clearly state that catching the Snitch ends the game, regardless of how many points the Seeker's team has.

Is it mentioned anywhere whether it is specifically prohibited for a Seeker to help the Chasers score points so that when their team is behind by more than 150 points they aren't essentially useless? Or would the Seeker just try to distract the other team's Seeker at this point?

Answer

In Quidditch Through the Ages, it is said:

However, a fast Keeper may be able to score a goal and then return to his baskets in time to prevent the other team equalising.

That's as much (that I've found) that's explicitly stated, but since the keeper can score, it's likely that a seeker or beater could as well. As for whether someone else could catch the snitch:

A game of Quidditch ends only when the Golden Snitch has been caught, or by mutual consent of the two team Captains.

It seems the rules don't say anything about who has to catch the snitch, it's just in a team's best interest to have a seeker searching for it full-time.

And, somewhat of a sidenote but I looked it up so I'll add it, in the sixth book, when Harry is in detention with Snape for using sectumsempra on Malfoy, Ginny replaces Harry as seeker:

And while he copied out all their various offenses and punishments, he wondered what was going on outside, where the match would have just started... Ginny playing Seeker against Cho...

(HBP, ch.24)