I got it working now, but just in case anyone else was wondering:
STEP 1:
in admin area you need to run this SQL-query on Patch page:
insert into xcart_product_tabs values (100, 'New Tab', '', 'Y', 'A', 4);
If you want to create more tabs with TPL use SQL-queries in this format:
NOTE: parameter #5 must be unique (S, D, U, R, C latters already are used).
insert into xcart_product_tabs values (101, 'New Tab2', '', 'Y', 'B', 4);
STEP 2:
in skin1/modules/xCartMod_Tabs/product_tabs.tpl find this line:
{elseif $tab.type eq "D"}
and put right before it this code:
{elseif $tab.type eq "A"}
<p />
{include file="modules/Detailed_Product_Images/product_images.tpl" }
NOTE: "A" - is unique modifier, modules/Detailed_Product_Images/product_images.tpl - any required TPL-file