The 19-year-old defender is out of contract after failing to force his way into the senior fold at Goodison Park.
His only experience to date has come during a loan spell at Stockport County.
The Sweden U19 international is, however, considered to be a decent prospect after spending three years on the Toffees' books.
Sides from across Europe are said to be keen to put terms to him, with Hammar's agent revealing that an extended stay in England is possible.
Martin Klette told Fotbolldirekt.se: "In England, there is interest from clubs in the Championship.
"In other countries, the interest is from clubs in the top flight.
"We are negotiating with some clubs and some of them want Johan to come for a trial."