Not quite. There is a grammar problem in the middle of the sentence.
You can't say 'if it's possible that you waive'. One way of correcting it would be this:
"I'm writing to inquire whether it's possible for you to waive the registration fees for us"
Now it's correct, but still informal.
Two ways of making a request sound more formal and more polite are to remove the contractions and to make the sentence a conditional, for example:
"I am writing to inquire whether it would be possible for you waive our registration fees"