At first I was reluctant to answer this question, as I believe in order to do this properly, will cost more than the value of said sub-woofer.
However as nobody else has commented, I will give you an answer on how you can do this.
There are two problems we need to overcome:
- The mixer has no further outputs to connect the subwoofer.
- The subwoofer has unbalanced inputs, which seems to be at "Consumer Line-Level
So now we need to do the following:
- Take the mix-output from both Alto TS210's, to feed the subwoofer.
- "convert those from "balanced / professional line level" to "unbalanced / consumer line level"
As a picture is a thousands words, something like this:
As you can see, I have drawn two "box's" in the picture, which will have to do the conversion.
This can be done with some equipment, and here is an example:
Using this example you can use input-1 and output-1 to do exactly what you need.
However the cost of this type of equipment may be more expensive than actually buying a subwoofer with proffessional line level inputs/outputs, which actually does that without additional equipment.
There are plenty of examples of alternative subwoofers which will do this, let me provide one here:
You would use "input" on the subwoofer to feed from the outputs of the mixing desk, then use "high pass out" on the subwoofer to connect your Alto's.
(I have no affiliation with either website or products mentioned in this answer, they are given as samples only)