CCP, Section 22(a) & (c) specifically say "if after a hearing" and "on a showing of good cause". I can't find any language in the code or case law that allows a defendant to simply agree to having his/her probationary term extended and skip any hearing.....but we do it all the time.

I believe the defendant can agree. Best practice is probably to file a motion to amend conditions / extend the term of supervision, and have the defendant waive service of the motion and waive attendance at the "hearing". Then, at the next docket call, just put a statement on the record that the extension is needed for good cause, and that the defendant has agreed to the extension. Then you have essentially complied with the text of the statute. These usually seem to come up in restitution cases where the defendant needs more time to pay, so they are agreed. Hope this helps.

Look at Tex. Code Crim. Proc. Ann. art. 1.14(a) (West 2011).

We file a motion to revoke and have the defendant come to court. The probation department usually have the extension paperwork done before the case is called, so the hearing consists of the judge asking "I've got this paperwork you've signed, Mr. Smith. Is that what you want to do? Ok, then..."

We treat an extension on probation as we would any other plea on an MTP or MTR. They fill out the same paperwork and the recommendation from the State is to extend the Defendant's probation for an additional X months rather than a revocation and jail time.