is there a way to add SLES 11 SDK repositories without having to download and burn the ISO ?
It is actually possible. I followed the steps at https://www.novell.com/support/kb/doc.php?id=7015337 and it seems (at least for now) that the repositories are activated.
In my case, I wanted the SLE11SP3 version of the SDK, which matches the article; if you are interested in a different version, you might have to do a bit of guesswork to find the right RPMs for your release (assuming they're available at all).
Thanks to the link shared by @koniiiik, I found out that in YaST2 you can add following repository for accessing packages of SLES 11 SDK without downloading it all in an .iso: https://nu.novell.com/repo/$RCE/SLE11-SDK-SP4-Pool/sle-11-x86_64 (change "SP4" if you have a different SP).
When asked for credentials insert the ones you can find on SUSE Customer Center > Organization > Organization Credentials.
I don't know if you need to register your SLES or not for this to work (I did it).