RMSD reference in Analyze Trajectory protocol

Hello,
I have 4 output files from 4 dynamics productiuon runs, each 0.5 ns. When it comes to analyzing trajectory, I am using the first frame of the first production run as the RMSD reference. It works for the first output file, but how do I specify this frame in the subsequent three output structures, since it is not included in those structures. I have to analyze all the 4 generated trajectories together in the end, using the starting structure as reference frame.
Would appreciate any help or suggestions.