I think it turns out that MS Word, PowerPoint, etc. supports to directly enter a Unicode character (by using their char code etc.), but I think this technique does not work in MS Excel, so you got to use copy paste method, e.g you may insert your unicode chars into MS Word document, it will convert it into diacritical glyph, now you may copy the symbol from MS Word/or other sources to paste it into Excel cell. So, while inserting data as unicode chars directly might not work even using Aspose.Cells APIs, e.g Cell.PutValue() method.
If you have some other way around for inserting directly unicode chars in MS Excel, let us know with a sample file having such data with details/steps, we will check it soon.