At present I see only one paper by John Meldrum and M. Samman: On endomorphisms of semilattices of groups (=Clifford semigroups) from 2005 in Algebra Colloquium 12, 93-100. A further paper is announced, but I cannot find it.
For the generalized case of a partial module(strong semilattice of modules) over a partial ring (strong semilattices of rings) , see Lemma 3.2 in the paper "Generalized Modules over Semilattices of Rings" by M.El-Ghali M.Abdallah.
Dear Ahmed Munir, this is true, but for which Clifford semigroups do all homomorphisms between the subgroups come from endomorphisms of the Clifford semigroup?
Dear Ulrich Knauer, A pdf of that paper is allowed in my home page on Research Gate.About your question,it is not that easy to extend a homomorphism between two maximal groups in a Clifford semigroup(semilattice of groups) to the Clifford semigroup except under certain conditions on the connecting mappings.Actually the problem is that,if wee have a collection of homomorphisms between the maximal groups in a Clifford semigroup indexed by its semilattice of idempotents ,how can this colliction be extended to a single endomorphism of the Clifford semigroup?