SLES and OpenSUSE are two very different species. SUSE is extremely concervative and has very limited package selection.
What follows may affect your SUSE support contract. SUSE support has repeatedly asked to remove any "foreign" packages, so if you follow instructions below, you might end up having a bad day when calling to SUSE support.
Some OpenSUSE packages will run happily on SLES without modification. What you can do is to enable an OpenSUSE repository for zypper and then try to install mosh from it, just to see if it works or not.
To get one of OpenSUSE repositories into zypper you create a file,
/etc/zypp/repos.d/OpenSUSE-repositories.repo with following content:
When you've created the file, just run
zypper ref OpenSUSE-11.4-OSS and then
zypper search mosh.
This example uses OpenSUSE 11.4 and enables only one of the repositories, namely Open Source Software (OSS) repository. Adjust the version to match your SLES version as close as possible (going lower usually works, going higher will not).
You can find list of OpenSUSE repositories here.