Swift 3, 51
Dependency: Foundation for String(format:_:)
{$0.utf8.map{String(format:"%02d",$0-96)}.joined()} // where $0 is String
Usage:
"codegolf".utf8.map{String(format:"%02d",$0-96)}.joined()
or
{$0.utf8.map{String(format:"%02d",$0-96)}.joined()}("codegolf")"