Portable Add-ons

Did you try to just copy them over from your local profile to your portable one?
What addons do you want to copy?

I don't use SeaMonkey but it's related to Firefox so it should be possible.

I have used MozBackup in the past to copy data (bookmarks, add-ons etc) from a (locally) installed copy of Firefox to a portable copy, and MozBackup also works with SeaMonkey so that is my first recommendation.

http://mozbackup.jasnapaka.com/

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If you want to do it manually, or only want to copy the add-ons, this page looks promising (if you're using SeaMonkey 2)

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_SeaMonkey

the bit about moving extensions

extensions folder - Contains added themes and extensions. Important! DO NOT ATTEMPT to transfer themes or extensions from a SeaMonkey 1.x profile to a SeaMonkey 2.0 profile. Extensions and themes in 2.0 are handled completely differently. If you are transferring data from a SeaMonkey 1.x profile, you will need to install your extensions again.
Not recommended, even for transferring between SeaMonkey 2 profiles, if you had problems in the other profile. After copying this folder or its contents to the new profile, delete the "extensions.ini", "extensions.cache" and "extensions.rdf" files from the profile folder. These files will be regenerated. Note that some extensions store data in additional files in the profile, usually at the top of the directory. You'll need to move them separately.

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Good luck