I want to extract the u and v fields from the FNL archive according to the pressure level in some locations. Is this possible?

You can extract a regional subgrid from any ARL packed data set using the utility program “xtrct_grid” in the \metdata subdirectory. However this program will extract all fields. The purpose of these programs is to simplify Hysplit simulations rather than provide general meteorological support. There are no programs that permit just the extraction of one or two fields. You can write your own programs to do that using the source code provided in the \metdata\source directory using the “chk_????” programs as a starting point. This process does require a thorough understanding of the structure of the packed data. Much of this detail can be found in the user’s guide.