Kevan told the Nottingham Evening Post: "Phil has done very well for us and has settled in very quickly at the club. We are pleased that can continue.
"Securing a deal for him to the end of the season gives us a great chance to assess him even more.
"He will still be under contract at Chesterfield in the summer so we will need to address that nearer that time."
Picken will still have 12 months remaining on his contract at Saltergate at the end of the season.