I had the same problem/question about two weeks ago. The previous post was what I used to do before I bought Architect. I wanted to utilize some of the features in the automated titleblocks but still use my custom block. This is what I did.
I typically use the same sheet size every time, archD. I went in and modified the and modified the title block object that VA setup pulls from. This can be found in vectorworks/toolkit/obects. Open 00_Arch_Sheets.mcd. Use resource pallete to select sheet/symbol to modify, select edit. Make your changes. Exit symbol. Exit drawing and save. I suggest saving a copy of the original 00_Arch_Sheets.mcd to revert to if you screw the whole thing up.