Function-Based Index on numeric Column

Question

I have a table containing some numeric columns, and i have to keep them numeric because most of the time i will benefit from that. But i need also to make a generic search on those columns using partial matches so in my where statement i will have something like

...where num_col1 like "1234%...

My question is:

Can i make a function based index on num_col1 casting the column to CHAR? I tried but it seems it is not possible.

If no, do you guys have any other suggestions on how can i speed up the results of my queries?

I read around that some possible solutions could be either to create a View of my original table and there change the column type to varchar and index that column or another solution could be to add an extra varchar column to my table and index that. I would like to avoid both of these solutions because i have a really big table containing already a huge amount of rows and huge amount of columns.

Thanks in advance to everybody, Best, N.