Can gold be added to silver to complete the nisaab?

Firstly: 

Zakaah is due on jewellery according to the correct scholarly
opinion. This has been discussed previously in the answers to questions no.
19901 and
59866. For information on the
nisaab for both of them, please see the answer to question number
64. 

Secondly: 

If a person owns both gold and silver, and neither of them
reaches the nisaab, but when added together they do reach the nisaab, he
does not have to pay any zakaah. Similarly, if a person has 70 g of gold and
400 g of silver, he does not have to pay zakaah on them until one of them
reaches the nisaab, because of the hadeeth of Abu Sa‘eed (may Allah be
pleased with him): “There is no zakaah due on less than five wasqs …”
Narrated by al-Bukhaari, 1405; Muslim, 979. And because gold is not the same
type as silver, so they cannot be added together, just as other types of
things cannot be added to one another to complete the nisaab. Camels cannot
be added to cattle, cattle cannot be added to sheep, wheat cannot be added
to barley and dates cannot be added to raisins. 

An-Nawawi (may Allah have mercy on him) said: Gold cannot be
added to silver or vice versa to complete the nisaab. There is no difference
of scholarly opinion – in our madhhab – just as dates cannot be added to
raisins.

End quote from al-Majmoo‘, 5/504. 

He also said: The nisaab for dirhams (silver coins) cannot be
completed by adding gold or vice versa, even if one owns two hundred less
one dirham, and twenty mithqaals less half a mithqaal and so on. There is no
zakaah on either of them. This is the opinion of the majority of scholars
and it was narrated by Ibn al-Mundhir from Ibn Abi Layla, al-Hasan ibn
Saalih, Shareek, Ahmad, Abu Thawr and Abu ‘Ubayd. Ibn al-Mundhir said: And
al-Hasan, Qataadah, al-Awzaa ‘i, ath-Thawri, Maalik, Abu Haneefah and all of
ashaab ar-ra’y said that one of them may be added to the other.

End quote from al-Majmoo‘, 5/504 

Ibn Qudaamah (may Allah have mercy on him) said in
al-Kaafi: Gold cannot be added to silver to complete the nisaab because
they are two different types. … And it was narrated from Ahmad (may Allah be
pleased with him) that he added them together because they are used for the
same purpose and in the same way. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The
correct opinion is the first one, that gold cannot be added to silver to
complete the nisaab except when they are used for exchange. That is because
wealth used in exchange is prepared for the purpose of trade, so (in this
case) the gold and silver are trade goods.

From Sharh al-Kaafi. 

He also said: Even though they are used for the same purpose
and in the same way, that does not mean that they can be added together to
complete the nisaab, because the Lawgiver has prescribed a specific nisaab
for each of them, which implies that zakaah is not due on anything less than
that, and there is no report from the Prophet (blessings and peace of Allah
be upon him) that he added one of them to the other. Just as wheat cannot be
added to barley to complete the nisaab, even though they are used for the
same purpose and in the same way, the same applies to gold and silver.

End quote from Majmoo‘ al-Fataawa, 18/248. 

And Allah knows best.