Yes, creatures can combine their strength to move an object
The rules for moving and lifting are not based on a check. They are automatically successful, if the character has sufficient strength to push, pull or lift the object (page 176 PHB).
Your Strength score determines the amount of weight
you can bear. The following terms define what you can
lift or carry.
When you push or lift normal objects in the real world with several people, the weight of the object distributes across them, so each of them only needs to push or lift their share. Everyday objects behave as we would expect them in the game, therefore the strength of serveral characters can be combined to push or lift a heavy object using the normal lifting and carrying rules.
So, "for a collective push, pull, or lift" the strength scores of the characters are effectively added together.
There also is precedent, for example from the 5e edition of The Hidden Shrine of Tamoachan in Tales from the Yawning Portal (page 67), where there is a large stone block the characters might want to move (emphasis added):
Moving the Block. (...) the block can be pushed inward by the combined effort of up to four characters with a total strength of 48 or higher.
In your example with the door, the issue is that you are modeling opening the door as an Athletics check with a DC, which is different from moving or lifting an object. You can avoid this problem by presenting the door as a weight to be moved, or as the required Strength to spare the DM some calculation, stating that X characters can combine to achieve a strength total of Y to force the door open. In doing so you lose the uncertainty created by a die roll that a check has, which may be OK for you or not.
If you are looking for a specific rule or rule variant that enables combining strength scores in a situation that calls for a check, there is none.
I agree with Dale M's answer, that this is easily within the range of things where the DM could make a ruling to maintain verisimilitude, for example by ruling that each helper adds some of their strength to the total strength used for the check.