My backed up wallet is from my phone and doesn't offer that extension.
Is there a way to convert my backup wallet which was produced from a cellphone into a wallet.dat file on a desktop?
Mostly not directly. What wallet did you use to make this backup? And what 'extensions' of this backup are available to choose from??
I think it basically depends on what kind of backup this is and
how it got encrypted. Depending on the length of the password the decrypting could take quite some time.
If you remember some characters of your password and/or had a short PW it might be possible, but with 8-9+ characters and additional chars this isn't doable (yet).
But we might help you out more if you would give us some additional information like wallet software used.