Taken from: http://forums.microsoft.com/MSDN/showpost.aspx?postid=420884&siteid=1

Step 1: Dis-assemble the assembly
ildasm myTest.dll /out:myTest.il

Step 2: Re-Assemble using your strong-name key
ilasm myTest.il /res:myTest.res /dll /key:myTest.snk /out:myTestSN.dll

This code work perfectly to assign strong name.

for verification you can use following command,
sn -vf myTestSN.dll