Many extensions are already compatible, but just need the necessary files or modifications to let them install in SeaMonkey. That's the kind of task I'm able to do.
I'm not familiar with how extensions themselves work, so if the extension needs its coding modified, Philip Chee'll have to come to the rescue. Even then, though, if the extension relies on functions that have no equivalent in SeaMonkey, it probably can't be ported.
Essentially, we'll try, but results are not guaranteed. Don't let that stop you from suggesting an extension, though.

Oh yeah, please post a link to the extension's AMO or home page if possible.
If the extension installs, and is advertised to work with Mozilla/SeaMonkey, please complain to the author, as it's their responsibility.
List of past requests:
Add to Search Bar
AutoLogin
BackToTop
Bookmark Duplicate Detector
Bookmarks Full Titles
BugMeNot
Google Toolbar
CacheViewer
Customize Google
DerBrowserTimer
Download Embedded
Download Statusbar
EasySearch Personal ToolBar Version
FareCompare
FEBE
File Title
Fission
Flat Bookmark Editing
Forecastfox
Formfiller
Foxmarks Bookmark Synchronizer
GameFAQs Toolbar
Gmail Space
Infolister
Linkification
MAF
Mail Redirect
mailTagger
miniT Tabscroll
OpenSearchFox
Paste Email
Performancing
ProfilePassword for Firefox
QuickQuote
Reload Every
Restart Firefox
Sage
Save As Image
Split Browser
Sun Cult
Surfulater
Tab Effect
Text to Image
Tidy Browser Button
undoclosetab
Unhide Passwords
Videodownloader
View formatted source
View Source Chart
Webmail