S-PLUS input facilities are simple and their requirements are fairly strict. There is a clear presumption that rather than use S-PLUS to accommodate a subtle variety of input, (and more so output), protocols you will be able to modify your input files using other tools, such file editors and the UNIX utilities sed and awk to fit in with the requirements of S-PLUS Generally this very simple.
There is, however, a function make.fields() that can be used to convert a file with fixed width, non separated, input fields into a file with separated fields. There is also a facility count.fields() that will count the number of fields on each line of such a file. Occasionally for very simple conversion and checking problems these may be adequate to the task, but in most cases it is better to do the preliminary spade work before the S-PLUS session begins.
Once a data set has been read, there is an X-window based facility in S-PLUS for making small changes. The command
xnew <- data.ed(xold)
will allow you to edit your data set xold using a spreadsheet-like environment in a separate editing window, and on completion the changed object is assigned to xnew. xold, and hence xnew, can be any matrix, vector, data frame, or atomic data object.