Yes, you can add a &custom= parameter to all of your E-junkie Add to Cart (or Buy Now) button URLs on the relevant site, like so:
<a href="https://www.e-junkie.com/ecom/gb.php?c=cart&i=100100&cl=20545&ejc=2&custom=sitename" target="ejejc" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this);"><img src="http://www.e-junkie.com/ej/ejadd_to_cart.gif" border="0" alt="Add to Cart"/></a>
Note that all buttons on a given site must use the same &custom= value, but if you use E-junkie to sell on multiple different sites, you could use a different &custom= value for each site. This value will be passed back to you in your Sale Notification email and Transaction Log, as well transmitted as your Integration URL and/or keygen URL (if configured).