Type menu>Insert special character>other>tab
Use the Tabs panel to manually create tab stops in table cells.
Under Mac OS: use [Alt]+[Tab] in a cell.
You can set a shortcut:
Edit -> Keyboard Shortcuts
Choose: Font menu -> Insert special characters: other: tab
Important! Context: Tables
With this (your own) shortcut you can now always insert a tabulator in a cell without the menu item.
Are these decimal numbers, or just integers? As I recall, the tab stop is implied in the cell, so you can actually set, for example, a decimal aligned tab as part of the paragraph style for the cell, then just type your numbers. When you type the decimal point the numbers should jump to position. Doesn't seem to work with left, center, or right tabs, but you can set positioning for that usually using alignment and insets.
This might not help if you need to have empty space between parts of the string in a single cell, however.