Lyman O. Littlefield states that Joseph "understood and believed" plural marriage in Kirtland.
Lyman O. Littlefield, “An Open Letter Addressed to President Joseph Smith, jun., of the Reorganized Church of Jesus Christ of Latter-day Saints,” April 27, 1883, in The Latter-day Saints' Millennial Star 45, no. 25 (June 18, 1883): 387
The doctrine of celestial marriage, I have the best of reasons for believing, was understood and believed by him away back in the days when he lived in Kirtland, when he and the Saints, in their poverty, were toiling to erect that sacred edifice wherein you now falsify him, seeking, by your unsupported declarations, to nullify his most sacred doctrines. Even there, as I believe, he was instructed of the Lord respecting the sacred ordinance of plural marriage; but he was not required to reveal it to the Church until some time during the residence of the Saints at Nauvoo, where he received a revelation from the Lord setting forth in detail the results to be obtained by keeping inviolate all the laws connected with this sacred condition of things.