If a frog is gradually exposed to increasing concentration of salts can they acclimatise or will they die of exosmosis ?
It will depend on how quickly the salinity is raised and to what level. Two studies (Karraker 2007, Karraker et al. 2008) looked at the effects of road salt (used to reduce icing on roads in winter) in the spotted salamander, the wood frog (a small frog), and the green frog (a big frog). Their studies showed that survival decreased, especially for larval individuals, as salt concentration increased. Deformities in embryos and larvae increased with salt concentrations$^1$.
Another study (Shpun et al. 1992) showed that a few species of frogs and toads that could tolerate high levels of blood plasma urea concentrations could also tolerate elevated levels of NaCl in their environment (but not all species that had high urea tolerance could tolerate high environmental salt concentrations.)
There would naturally be an upper limit of salt tolerance in amphibians, especially if they are not able to retain or regain water at a rate they are losing it to the high salinity environment. The studies suggest that higher salinities do decrease survivorship and increase deformities. While they may acclimate to the short term, I would question long term adaptation unless the rate of salinity increase is very gradual.
Karraker, N.E. 2007. Are embryonic and larval green frogs (Rana clamitans) insensitive to road deicing salt? Herpetological Conservation and Biology 2: 35-41.
Karraker, N.E. et al. (2008). Impacts of road deicing salt on the demography of vernal pool-breeding amphibians. Ecological Applications 18: 724-734.