If you are visualising a bunch of .vtk files that were generated separately from the standard fields in OpenFOAM, and if you are a lucky person like me at this point (your time steps are constant), there exists a simple way to visualize the files alongside OpenFOAM data:

1) Open the OpenFOAM case as usual.
2) Open the .vtk files. The files have to be named appropriately to be read as time-dependent data: someName-001.vtk, someName-002.vtk, guess what is the next name.
3) Use the temporal shift scale filter to scale the time interval of the .vtk files to correspond to OpenFOAM time steps interval. E.g. if you have 500 .vtk files, and your time interval is [0,0.5], scale the .vtk time interval using the temporal shift/scale filter by 0.001.

This tip was provided by Takuya Oshima on this thread:

http://www.cfd-online.com/Forums/ope...tml#post332933

Give the man some reputation points if you find this useful!