You can get cell's HTML-element with:
grid.getCellNode(rowIndex, cellIndex)
You can then edit the title of a cell with:
grid.getCellNode(rowIndex, cellIndex).title = "foo"
If you are using jQuery you can also do:
$(grid.getCellNode(rowIndex, cellIndex)).attr("title", "foo")
If you are using SlickGrid's AutoToolTip-plugin, editing the titles of cells won't work, because the AutoToolTip-plugin overwrites the title always when hovering a cell.