Not sure what your plan is for the addition but have you considered french doors with or without privacy glass? This would ease the pain of framing in 3' and make nice entry to the addition!

saskatchewan girl,
Thanks for the suggestion.
I don't want swinging (or bifold or accordian) doors because I want to maintain the same door/access schemes as in the existing house. Also, these types of doors would interfere with wall space and/or other doors.
The addition includes a laundry/utility room, foyer, and master suite.

--should I assume that a secondary header is already installed?--

No. A sliding glass door runs on a track on the floor and a pocket door on an overhead track.

Hendricus,

What sort of idiot would frame a load bearing wall for a 6' opening and not put a header in? In 30 years there should have been some sort of sag if there was no header.
canishel,
exactly what do you mean by secondary header? If there is a 6' sliding door now why would it not be framed with a full header? If you measure the inside of the jamb of the existing door and add 1 1/2" you will get the minimum opening that there could be, and possibly more if they shimmed the door jambs. You wouldn't know if it is shimmed unless you take off the trim. I would suspect the jamb is 3/4" per side. If done correctly, a rough opening should be about 3/4" more than the outside measurement of the door.

---Also, in a reply to a question about installing pocket doors, sierraeast wrote
"The track should also NEVER be attached to a header in a weight bearing wall.
A secondary header should be installed with about a 1/2 inch clearance below the structural header. "---

decker; this is from the original post, did you read that?

To all reading this post:
First of all, the quoted post should be attributed to Brickeyee. My sincerest apologies for my error. Here's a copy-and-paste version with the relevant section.

Posted by brickeyee (My Page) on Thu, Jan 31, 08 at 19:46
....
The track should also NEVER be attached to a header in a weight bearing wall.
A secondary header should be installed with about a 1/2 inch clearance below the structural header.
If the header bows (and they all do to some extent) it can cause problems with the track.
If the wall is NOT weight bearing the header will not be loaded and you CAN omit the second header.
Decker,
You asked what I meant by a secondary header. The real answer is I don't know; I'm just parrotting what I've read. But the term was used when describing 2x4(?)as something to which the pocket door track would be attached.

BTW, this door installation is not a DIY project. I'm just trying to second-guess potential problems in construction.

Again, I apologize if I've opened a can of worms.

Any updates on this project? Photos of success? My wife and I want to do this in our home, but can't find any good examples.Thanks!

A 30-year-old header has done all the bowing it's ever gonna do. Shim the track to the header to level and forget about it.

How about a sliding barn door? They look nice is some houses and can slide over to one side and be out of the way.

It's been 6 years and you haven't competed the project?

Thanks to all who responded.

Renovator8,
Yes, the project was finished years ago. The plan was posted to answer yourarockathome's request.