Only under conditions
The International Committee of the Red Cross has a lecture on the Law of Armed Conflict. Lesson 8 - neutrality, page 10, reads:
TRANSFER OF POWS INTO A NEUTRAL STATE
By arrangements between the States concerned (i.e. Parties to the conflict
with the co-operation of neutral States), POWs can be admitted and
interned on neutral territory until the end of hostilities. This rule caters
for those States that might not be able to provide POWs with the treatment
they must receive under the Third Geneva Convention, e.g. incurably
wounded and sick or mental cases. Agreements between the belligerent
parties and a neutral State mutually agreed by them, may also be drawn
up to cover the transfer of able-bodied POWs who have undergone a
long period of captivity
This is sourced to GC III, Arts. 109-111, of which Art. 111 reads:
Article 111 - Internment in a neutral country
The Detaining Power, the Power on which the prisoners of war depend, and a neutral Power agreed upon by these two Powers, shall endeavour to conclude agreements which will enable prisoners of war to be interned in the territory of the said neutral Power until the close of hostilities.
So not only is the offer allowable, if both Ruritania and Evilstan agree, it is actually encouraged under the Geneva Convention to find accomodation for any PoW that qualifies under Art. 109 and 110. The commentary on it reads:
4326 Article 111 should be read in conjunction with Articles 109 and 110. The latter two articles specify certain categories of persons who are eligible either for direct repatriation or for accommodation or internment in neutral countries during hostilities. They primarily cover wounded and sick prisoners of war, providing for the repatriation or accommodation in neutral countries only of those ‘able-bodied’1 prisoners of war who have been in captivity for a long time. Article 111 supplements the obligations in Articles 109 and 110 by requiring belligerents also to endeavour to conclude agreements with a neutral Power to intern all other ‘able-bodied’ prisoners of war in that country.
Conclusion
If Ruritania and Evilstan can agree that Neutralia is a good target for PoWs that are either in long detention or otherwise in conditions that can't be treated in either country, then the offer is in line with Geneva Convention III, Articles 109 to 111. If the offer is meant to accept all PoWs and not just those interned for a long time or in critical conditons, then it would be too broad.