Hi@yongsm,
Based on the issue that you mentioned, do you want to allow the users to fill in other choices within the drop-down control?
Could you please share a bit more about the scenario: do you want to fill in within PowerApps or fill in choices within the SP Choice column and display the added choice within PowerApps?
Let me make it easy for you, only the Combo Box could allow users to fill in text, so I assume that you use the Combo Box.
In the Combo Box, we name the fill-in text as SearchText.
If you want to show the fill-in choice in the Combo Box, you should enable the 'Can add values manually'. This operation could help you enter choices within the Combo Box.

Only this way could make the fill-in choice display within the PowerApps and allow the users to fill in choices whenever in the SP list or in PowerApps.
I have made a test which is confirmed on my side.
1/ If you have already added new choices within the Choice column in the SP list and just want to display the fill-in choice within PowerApps, you should enable the 'Can add values manually' function.

2/ If you just want to fill in a choice in the Combo Box within PowerApps, you should enable the 'Can add values manually' function within the SP list column setting and enable the 'Allow searching' function for the Combo Box.

Further, if you have the ComboBox within a Form, you should write the Update property of the data card as follows:
If(
IsBlank(DataCardValue10.Selected),
{Value: DataCardValue10.SearchText},
{Value: DataCardValue10.Selected.Value}
)
Note: DataCardValue10 represents the Combo Box.
Hope it could help.
Best Regards,
Qi