No, there's no way to have one in the Signature area. Some have been using a copy and paste into the main body of the psot to get it all in, but it can be confusing to those who aren't aware what is happening.
Looking at your Sig., there's lots of excess things that can be dropped or shortened to abbreviations to allow you more letters to go in the 400 limit. Fiddle with it and see what you can come up with. For abbreviations, look at the top of the main page to the sticky of Abbreviations to help with forming a more concise signature.
James C. Age 63
Gonna Make Myself A Better Man tinyurl.com/28e8qcg
4/07: PSA 7.6, 7/07 Biopsy: 3 of 16 PCa, 5% involved, left lobe, GS6
9/07: Nerve Sparing
open RP, Path: pT2c, 110 gms., clear except:
Probable microscopic involvement-left apical margin -GS6
3 Years: PSA's .04 each test until 04/10-.06, 09/10-.09, 01/11-.09