as we all know that the slice theorem is very important in symplectic geometry , especially in the proof of marstensternbergweinstein reduction theorem . so I wonder a similar question that does there is a similar theorem when the symplectic manifold have some sigular point ? and i need a original proof of the slice theorem 'Sur certains groupes de transformations de Lie' by Koszul , if you have the electric version of this paper,please send a copy to me wangzhiwei08@gmail.com.
I am only aware of results concerning singular symplectic reduction, i.e. when the Hamiltonian group action is not free and the quotient space is only a stratified symplectic space. The theorem is due to Sjamaar and Lerman ("Stratified symplectic spaces and reduction"). 


I'm not sure if I understand your question correctly, but
extends the notion of a slice to the case where the underlying space is not a manifold but only a stratified space. They also proof a slice theorem in this setting, but this is clearly adjusted to the discussed case of General Relativity. 

