Since "You can specify all of the logical expressions in the same way that you can specify them for the INCLUDE statement....", look at the "Numeric Tests" under the "INCLUDE Control Statement" section in "Chapter 3. Using DFSORT Program Control Statements" in "DFSORT Application Programming Guide".
fo example: "INCLUDE COND=(1,20,FS,EQ,NUM)"
Here's a DFSORT job that will do what you asked for. I assumed you wanted PD zeros for non-numeric values. You'll need z/OS DFSORT V1R5 PTF UK90007 or DFSORT R14 PTF UK90006 (April, 2006) in order to use DFSORT's NUM function. If you don't have the April, 2006 PTF, ask your System Programmer to install it (it's free). For complete details on all of the new DFSORT and ICETOOL functions available with the April, 2006 PTF, see: