Insert mutliple column vaules into single column in another table

Experts,

I have an extremely large table that I need to get the values of 10 different columns and insert them into a single column in another table. I suppose I could use a UNION query but I'm assuming there is a more efficient way.

As an example, lets say I have a table T1 and I need Col1, Col2 & Col3 data and I need to insert them into another table T2, into a column called Member_ID.

So instead of the UNION query is there something more efficient since I have 10 columns to get and @60 million rows to go through?

Thanks for your time.