nakamuraza, in answer to what I believe you are asking -
it's completely normal to include past work on your showreel, not just your current agency or company.
if you prefer, bring up a title Work with Saatchi & Saatchi and have that, and then bring up another title Work with previous employers and then have that.
No big deal.
It's obvious that those in the video/film industries of course include all work they have done, on their showreels.
If you are unfamiliar with this you can instantly:
google up perhaps 1/4 million such reels
instantly see that reels / folios include all their work.
BTW for any young people reading, this is a (physical, not metaphorical) portfolio as everyone on Madison Ave. would carry around up until, say, 2000 or so..
...for those who worked in print advertising. For those who did TV spots, this is a (physical, not metaphorical) showreel.
(TBC note that the OP does not at all mention "reel" or "showreel" - I just added it here for clarity.
The OP states twice business portfolio, simply meaning: "When I assert my body of work includes A B C, can I include X Y Z ?" )
Note that the words "portfolio" and "reel" are also used metaphorically. Eg, "Steven Spielberg's reel includes ET and Jaws." (The guy wouldn't literally bother having a physical tape somewhere with those on it - you know, so that he can show it to potential employers :) )