This is dependent on whether or not a .xlt file is a valid .xls file. If
it is, then you can just use this code:

FileInputStream in = new FileInputStream("C:\\blah.xlt");
FileOutputStream out = new FileOutputStream("C:\\blah.xls");
int byte;
while ((byte = in.read()) != -1) {
    out.write(byte);
}

If not, then tell your problems to Microsoft.

Well I have been up to doing such things some time ago, and I dug up
following things:
1. Doing it witg Office is not a good idea, first you may break the
law (by de-enginierring closed specyfication), second you won't get
any help from nobody except for money, third your spreadsheet may look
awfyully disorted when you change version of microsoft office (well
office is not fully compatible with itself), last it's awfully
complicated.

2. Depenmding on what you want to do you have some open--source free
software.

If you need beautifull noneditable documents use TeX output, then
(form your program) compile it to pdf. Learning to TeX is something
like two weeks of work.

If you need ugly editable documents use .csv (comma separated valuse)
format for spreadshets, and txt or rtf for documents. With csv you'll
get the perk that anything will be albe to render is as a spreadsheet
(Exel and Calc, Google Spreadsheets, even some obscure scientific
software).

If you need nice and editable documents use open document format. It's
open (so you'l get specyfiaction, and a lot of free help). They're
also working on ways to make such programs easier to use.