You seem to have picked up on the easiest pattern for this:
Weak masculine nouns end in -i, weak feminine nouns end in -a.
Weak nouns are easier because there's only one main pattern for each gender.
This is less universal for proper names, but in general, a weak masculine will end in -i, and a weak feminine or neuter in -a.
Nouns ending in -ir are the same for both genders.
These are the r-stems, almost exclusively words for family relations. Both masculine and feminine end in -ir: for instance, bróðir and systir ("brother" and "sister").
Strong masculine nouns usually end in r, strong feminine nouns usually don't.
There are several other sub-classes of strong nouns (a-stems, ja-stems, wa-stems, ó-stems, jó-stems, wo-stems, i-stems, u-stems, nd-stems, and consonant-stems), but if you only want the nominative singular, the details don't really matter. Almost all of these have a final -r in the masculine and no ending in the feminine.
(A final -nn tends to come from a stem ending in -n, plus the ending -r, by assimilation. So consider it to be -nr for these purposes. Similarly, read -ll as -lr.)
I-mutation!
Certain front vowels in Proto-Norse caused umlaut of the other vowels in the word. This is hard to predict in Old Icelandic, unfortunately, since most of the sounds which triggered this effect disappeared. But in general:
A final -i or -r, after a heavy syllable, may have changed the preceding vowel.
The changes are quite regular...the only question is whether or not they happened.
- a → ę
- á → æ
- o → ø
- ó → œ
- u → y
- ú → ý
- au → ey
- jú → ý
- ǫ → ø
(Table from Gordon's Introduction to Old Norse, p34/251)
So when you change the ending, you may have to apply this change, or undo it. Unfortunately there is no reliable way of deciding this without knowing the etymology of the name. (It depends whether the ending contained an i or not in Proto-Norse.)
U-mutation!
As if one type of umlaut wasn't enough! Vowels before a u or w sound tended to shift as well. And worse yet, this w often disappeared entirely after messing up the vowels.
- a → ǫ
- á → ǫ´
- e → ø
- ę → ø
- i → y
- ei → ey
(Same source, p40/253)
Examples.
The best examples for this are adjectives, since they have masculine and feminine forms on the same stem.
Strong: M langr F lǫng
Weak: M langi F langa
Notice the u-mutation on the strong feminine form.
The good news is, unlike the adjectives, there aren't certain etymologies or feminine forms for most of the dwarves' names. So you have some leeway in applying or ignoring the umlaut to make it sound better. See for example this answer, which feminizes "Mundilfęri" as either "Muldilfara", "Mundilfęra", or "Mundilfǫr". (I believe the "ä" and "ö" in that answer are equivalent to normalized ę and ǫ—please correct me if I'm wrong.)
