The pull request cannot be changed (it represent a tree you don't own), but you can checkout the branch that represents the pull request and merge a squash of all commits (git merge --squash).
Nevertheless, I would recommend not to do this: If commits were done separately, this probably mean that there were some incremental work done. You may think that, now job is done, the incremental view is useless, but this is exactly the contrary: if you experience a regression or if you find some awkward code in the future, you will probably try to find when and why the code was merged, and having small commits will help you to understand far more easily than a huge commit of 6000 lines saying "merge feature XXXX".
BTW reviewing small patches if far more efficient than a huge one (I don't mean it is faster, I mean that you find more problems when modifications are smalls and the intention of the one who committed clear).
That say, many people use the squash technique, thus I guess it can be seen as "local good practices", after all, a team decides how it wants to work together.