Sorry, no such modification exists. WC3 is a piece of computer software using DirectX 8 or OpenGL to display graphics. Both of these interfaces have no support for infinity when it comes to 3D positions (which are floats or double floats nowdays, both of which are restricted in size) and thus it would be impossiable to reach infinity. The maths for calculating the graphics for a triangle with its Z demension set to an undifined value would also be impossiable to physically compute. Thus what you are asking is physically impossiable for WC3 to support, I would advise looking for a game which does allow it however I doubt there is any as infinity and computers generally are not very good if handled improperly.
I can however tell you there is a modification which involves simply importing a custom text file into a folder into your WC3 directory which allows you in WE to ignore the usual heigh raising and lowering restrictions thus allowing you to raise or lower terrain to any value WC3 is capable of supporting, unfortunatly it is not infinity so it may not be what you are after.
The modification I described above involves modifying the MiscData.txt found in the WC3 RoC MPQ under the UI folder and changing the maximum terrain slops and related fields to high values so that the limates do not exist. You then make a folder in your WC3 install directory called UI and put the modified one in it and then restart WE. Like I said this has nothing to do with infinity but is the closest thing you will get to it.