No usually people will do this for friends who have privet SG's for storage. It gives them access to porters etc. I have done this for people and have had it done for me. Finding the sg willing to do it might be hard but i don't see it being wrong/rude at all to ask.
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Go ahead and ask politely. Generally, I'd start with people you know well already, rather than out of the blue.
www.paragonwiki.com is a great source of information for this game.
New or returning to the game? Want advice from experienced players who want to help YOU?
The Mentor Project: Part of the New Player Council.
Not rude at all to ask.
HOWEVER, if you're doing it for purposes of teaming, I think you will get better results from joining some global channels. You should post in the section for your server and ask for recommendations. Also, you can look in the Player Guides section, where there is a stickied thread List of Global Channels.
Paragon City Search And Rescue
The Mentor Project
Sometimes one man or two man sg's can play more often than those housing 50 alts with maybe one player lol. Never rude to ask for a coalition 
So I've recently returned to the game and have my homey little one man SG (well, 2 man I guess... me and my son). I like the convenience of having my private storage tables and such, but am interested in hooking up with a SG for some teaming and such.
Is it considered bad form to ask an existing SG to add a one man show to a coalition instead of outright joining them? I don't want to step on anyones toes or put forth something that is considered rude or offensive... so I figured I would check before I started poking around.