Steve Clarke is hopeful of completing the move by Thursday, when the loan window closes, and Appiah will link up with his new teammates that are not away on international duty.
The Ghana international impressed at the African Cup of Nations tournament in January and spent a spell on loan at Cambridge United earlier in the season, scoring six goals in 19 appearances.
The 24-year-old has found his chances limited at Crystal Palace and will hope that he can impress in the Championship after numerous loans at several League Two clubs over the past few years.
Clarke has moved to boost his forward line after Yakubu and Simon Cox missed the trip to Charlton with injuries picked up in training.
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