You can't as of yet. According to Google's support page, Rows and columns: Freezing columns and rows:
Freezing Columns
There are also two methods to freeze columns in a spreadsheet:
The Column Bar is a vertical bar that separates active columns from frozen ones. Using its handle (found at top, by the column headers), you can drag the Column Bar left or right to the last column you'd like to freeze, 5 columns maximum.
Click the Tools tab and then click the "Freeze columns" drop-down menu. From the menu, select the number of columns you want to freeze, 5 columns maximum.
