I do not see any reason not to do that, so long as it is done with the left hand.
Foreplay with one’s wife using one’s finger
Question 21676
Is it permissable for the husband to insert his finger into his wife's vagina and to fondle it during intercourse.
Answer
Praise be to Allah, and blessings and peace be upon the Messenger of Allah:
Kind Treatment of Spouses
Source:
Shaykh ‘Abd al-Kareem al-Khudayr
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