Alright, sorry all I figured it out finally. All I did is created a border variable of type BorderLine like follows
Dim border As New unoidl.com.sun.star.table.BorderLine
Then I set the outerWidth like so:
border.OuterLineWidth = 1
Then I use the setPropertyValue method for the specific cell that I need So something like this:
objCell.Properties.setPropertyValue("BottomBorder", New uno.Any(border.GetType, border))
Of course Properties between ObjCell and Properties is the xPropertySet of the cell. Anyway, hope this helps someone else.