The 29-year-old, Argyle's second longest-serving player behind goalkeeper and fellow Frenchman Romain Larrieu, trained with the Lions on Friday and has been given permission to play in their pre-season friendly against Middlesbrough at The New Den on Saturday.
Plymouth's out-of-favour midfielder Walton has also been training with Millwall this week with a view to a permanent switch. The 21-year-old could also feature against Boro.
"Mat trained with Millwall and we have given him permission to play in a game for them," confirmed Argyle boss Paul Sturrock.
"Walton has also been away with Millwall for the last three or four days. I don't know whether he will play for them. I haven't discussed it with Millwall yet."