It's actually very easy. Just add essentially the same thing after it with different names. Sound confusing? I'll give you an example.
Let's make the Germans have both Panzer and Landsknecht. Here, you have the CivilizationInfo.xml. You browse down and come across the info about the Germans. Browse down some more and you'll see the <Units> info. Now, let's experiment...
<Units>
<Unit>
<UnitClassType>UNITCLASS_TANK</UnitClassType>
<UnitType>UNIT_GERMAN_PANZER</UnitType>
</Unit>
</Units>
See those bolded parts in the quote screen? In XML language (that little that I know) it creates a folder inside the file within which the information will modify the civilization's unique unit properties. In this case, between the two bolds is added a replacement for the typical Tank. If we intend to add the UU Landsknecht after the Panzer, we just add another <Unit> </Unit> entry between the the <Units> and </Units> tags. Now, it'd look like something like this...
<Units>
<Unit>
<UnitClassType>UNITCLASS_TANK</UnitClassType>
<UnitType>UNIT_GERMAN_PANZER</UnitType>
</Unit>
<Unit>
<UnitClassType>UNITCLASS_PIKEMAN</UnitClassType>
<UnitType>UNIT_HOLY_ROMAN_LANDSKNECHT</UnitType>
</Unit>
</Units>
The info in bold is what I added after the Panzer entry. Notice how for each unit you add you need to have seperate <Unit> tags that must both start and end and be positioned between the <Units> </Units> tags, which help the system categorize each civilization's unit information as it needs to. Anyway, in this example, you must also take into consideration what unit class the new UU is going to be. Landsknecht replaces Pikeman in the regular game, so of course its unit class is Pikeman. Hwacha (Korean UU) replaces Catapult so its unit class is Catapult and so on...
And the UU's name itself, realize that it needs to have the civilization's civil adjective before its actual name. Holy Roman Landsknecht needs to be called exactly that, HOLY_ROMAN_LANDSKNECHT. GERMAN_LANDSKNECHT or HOLY_ROME_LANDSKNECHT won't do, it needs to be exactly the right name. However, the pattern is easily understood (civil adjective + unit name) so that's hardly a problem. What's important is fitting all this between the <Unit> </Unit> tags, which in turn must be fit between the <Units> </Units> tags.
Also remember, all the letters that I wrote as capital letters must remain capital letters and all the underlines (_) I wrote as underlines must remain exactly that. No spaces, no dots, no commas, just underlines. They're important when seperating pieces of text from one another. Nothing else matters as much as the last thing I said. Keep that in mind, hope that helped.