There's a new feature in VPHOT that lets you copy WCS parameters from one image to another. The WCS parameters is what makes up the plate solution, so in effect this means 'plate solving' an image by using another already plate solved image.You do this by listing the relevant images in the image list, select one that is plate solved and one (or more) images that are not plate solved. Then click the 'Update WCS' link in the menu above the list.
!!!WARNING!!!! Use this feature with great care!
It only make sense to use this feature with images taken close in both time and space. The telescope should not have drifted in the time between the solved and unsolved image. It will do you no good adding incorrect WCS parameters to an image. Consider this an emergency solution for solving the odd image in a series that for some reason did not plate solve.