You should initially test with a small amount of extract, how much water it needs to dissolve completely, if it is not possible to dissolve it in water. Then you would not be able to make the mixture with DMSO, as the insolubility in the DMSO/water mixture would persist.

I have not worked with cells, and what I do in in vitro molecular assays, is to prepare a stock solution with the same extraction solvent (70% ethanol), and from the stock I prepare dilutions in the test solvent (in this case it would be with DMSO). If the ethanol residues found in these dilutions do not affect cell viability, this could be a working option.

The other option, although I have never worked with it, is to prepare the stock dilution with DMSO-ethanol, i.e. the minimum amount of ethane that dissolves the whole sample and then complete a volume with DMSO. From these prepare the dilutions in DMSO. In this case you will also find traces of ethanol in the sample.

The last case would be to work directly with the aqueous extracts of the plant samples.

You can also change the use of ethanol to methanol. Again, cell viability should be checked.