Reading values from another transaction: How can we display in transaction MD14 additional data that are availabe in transaction ME13?

Normally the fastest way is to call up a BAPI function that returns the values you need.
If you prefer to use the ME13 screens, you need to save the data that the user has already entered, then call up ME13 in your InputScript with /nME13, read the values from the ME13 screens, return to MD14 and restore the input values. In some cases, if there is already a menu item that calls up the desired transaction in "call" mode, this procedure can be simplified in that you don't need to leave your main transaction.