After flying out of a grass field for some months I would like to add some sort of landing strip to the field - tarmac or whatever. Is there a program that will do this? And do it simply? Or is it simply not possible or overly complicated?
Thanks in advance for any suggestions..

It can be done quite easily. The software used depends on what FS version you are using.

For FS2004: (the same as suggested by Hager)
Afcad 2.21-Lee Swordy

For FSX: both of these require that you have the SDK, included with FSX-Deluxe, installed.
Airport Design Editor-Scruffyduck Software
FSX Planner-zBlueSoftware

All three programs are freeware. Each provides a 2D cad style window to create or modify the visible (runway, taxiways, aprons, parking, surface lighting, etc.) and invisible (comm and nav aids, etc.) aspects of an airport. The FSX programs can place some additional items specific to FSX airports.

If your using FSX-Standard (no SDK) I only know of one airport editor the does not require the SDK:
AFX-Flight1 Software-payware, demo available....for FS9 and FSX.

Dave